In order to determine whether the air-fuel ratio (AFR) is rich or lean, the ECU (engine control unit) in a vehicle with electronic fuel delivery control (either carburetor|carbureted with electronic mixture control, or with elecronic fuel injection/EFI) monitors the voltage output of an oxygen sensor (sometimes referred to as an O2 sensor) and adjusts fuel delivery accordingly in order to closely approach a stoichiometric ratio.
Dual exhaust refers to the exhaust system in vehicles, especially passenger cars or pickup trucks, which have two distinct exhaust systems to carry exhaust gases away from their internal combustion engine. This is most common on vehicles with V-shaped engines. It is both a common aftermarket performance enhancement and is often found on stock vehicles. It provides better performance than badly-implemented single exhaust systems because it allows for less resistance to exhaust flow, both due to the size of the piping and also because an exhaust system which has passages of inequal length before two pipes come together will have a resonant effect that results in uneven exhaust backpressure which changes at different RPMs.